Air filter



FIG. 1 is a perspective view of an air filter, showing my new design;

FIG. 2 is a front, rear, and left and right side elevational view thereof;

FIG. 3 is a top plan view thereof; and,

FIG. 4 is a bottom plan view thereof.

The broken-line showing of environmental structure is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for an air filter, as shown and described. 